In electrical power distribution systems it is frequently necessary to sense the occurrence of fault currents in three phase underground cables, which typically consist of three 120xc2x0 radially spaced conductors encased within an electrically conductive lead or lead sheath and an overlaying PVC jacket. The cables are typically run from vault to vault, the conductors being buried in the ground or routed through conduits between the vaults and being accessible through the uncovered vault or manhole.
Conventional current transformers cannot be used for sensing fault current in three phase sheathed cables because the sum of the balanced 3 phaser currents is zero, so that the net current sensed by the transformer is zero. Consequently, prior devices for sensing fault currents in such cables have been installed at cable splices or terminations, where individual conductors are accessible and the phase currents can be separately sensed.
The present invention is directed to a fault indicator which senses faults in a three phase cable without physical access to the individual conductors within the cables, thereby allowing the indicator to be positioned at any location along the cable, such as where the cable passes unterminated through a cable vault. For optimum performance in this environment, the fault indicator is moisture-proof and includes an aural alarm which, in the event of a fault, can be heard from outside the cable vault, thereby obviating the need for physical inspection of individual fault indicators along the cable.
Accordingly, it is a general object of the present invention to provide a new and improved fault indicator for sensing the occurrence of a current in three phase power distribution cables.
It is a more specific object of the present invention to provide a fault indicator which senses fault currents in three phase sheathed cables without the need for physical access to individual conductors within the cable.
It is another specific object of the present invention to provide a fault indicator which can be installed at any location along the length of a three phase sheathed cable.
The invention is directed to a fault indicator for sensing a fault current in a three phase three conductor sheathed cable of the type having first, second and third electrical conductors. The fault indicator comprises first, second and third magnetic pole pieces arranged primarily in the magnetic fields surrounding the first, second and third conductors, respectively, first, second and third magnetic windings in magnetic communication with the first, second and third pole pieces, respectively, for developing first, second and third signals indicative of the current level in the first, second and third conductors, respectively, indicator circuit means responsive to the first, second and third signals for indicating the occurrence of a fault current in the conductor.